The invention is based on a device for actuating a clutch according to the preamble of claim 1.
Devices for actuating a clutch that have an electronically controllable clutch servo unit are used for motor-vehicle drives having an internal combustion engine and a mechanical shifting mechanism capable of being shifted fully or semi-automatically. The clutch or clutches can be used to transition the vehicle to motion or to shift gears. Such a motor-vehicle drive is made known in U.S. Pat. No. 5,441,462 in which an electronic control unit controls a clutch servo unit depending on operating parameters and driving parameters.
Furthermore, a clutch servo unit for engaging and disengaging a vehicle clutch using an electric motor is made known in DE 197 01 739 A1, which said clutch servo unit actuates a rod via a reduction gear in the form of a worm-gear pair having a worm gear and a spur gear, which said rod is guided in axially displaceable fashion and acts on a master unit of a hydraulic transmission path to the clutch. In accordance with different applications, clutch servo units with different electric motors and reduction gears are to be developed, produced, and kept on hand. Moreover, long transmission paths feature numerous sources of error and result in inaccuracies in terms of the actuating motion.
A clutch servo unit is made known in WO 98/13614 A1 in which a motor shaft of an electric motor features a thread on which an actuator having an internal thread sits. The actuator is immobilized in the circumferential direction in the housing of the clutch servo unit, but it is guided in axially displaceable fashion, so that its axial position changes when the motor shaft rotates. The actuator acts on the throwout bearing of a clutch via a mechanical or hydraulic transmission path in the form of a lever mechanism or a hydrostatic system. The electric motor is assisted in the direction of actuation of the actuator by an accumulator spring that is mounted on the housing on the one hand and, on the other hand, acts on the actuator. The components of the actuating forces and forces of the accumulator spring acting in the axial direction are supported via the mounting of the motor shaft, so that the bearings must be strengthened and the bearing play and the tolerances of the bearing components influence the accuracy of actuation.
According to the invention, the gear element that is axially displaceable relative to the housing is interconnected with a rotor or a motor shaft of an electric motor via a longitudinal guide in a manner that is torsionally-resistant but axially displaceable and acts via an actuator on the throwout bearing. As a result, the actuating forces and, if applicable, the forces of an accumulator spring are supported directly by the housing via the gear element immobilized in the housing. The motor bearings are not loaded by these forces, since, except for negligible friction forces, no forces can be transferred in the axial direction via the longitudinal guide. Moreover, the tolerance chain between the throwout bearing of the clutch and the support is very small, so that tolerance errors in the actuating motion and the position of the clutch servo unit can be kept very small.
The electric motor of the clutch servo unit acts on the throwout bearing via a high-reduction actuator gear, so that no additional amplifiers or transmission paths such as a hydraulic path or a hydraulic pressure booster, for example, are required. The sources of error typically associated with these systems are avoided in the device according to the invention, namely, for example, gas accumulation and leaks in the hydraulic path, which, combined with an ageing of the hydraulic fluid, result in ageing-induced displacements of power.
The reduction actuator gear is advantageously formed by a spindle gear or a helical gear of a known design having a shallow-pitched thread, or a spiral band gear in which the turns of the thread are formed by a spiral band. Of the gear elements that are interconnected in terms of propulsion, one of them at a time is immobilized in the housing, while the otherxe2x80x94driven by the electric motorxe2x80x94is supported in torsion-resistant but axially-moveable fashion on the motor shaft by a longitudinal guide.
In the case of a helical gear, a gear element has an internal thread that meshes with an external thread of another gear element. In the case of a spiral band gear, a gear element is formed by a spiral band, while the other gear element is a transfer element that meshes between two adjacent windings of the spiral band by means of radially-oriented ends located on the circumference. When the electric motor drives the gear element interconnected with it, it rotates relative to the gear element that is held in the housing in torsion-resistant fashion. The gear element supported in a manner that allows it to move axially on the motor shaft is thereby moved in the axial direction by the thread. This actuating motion is transferred via an actuator to a throwout bearing of the clutch. Such an actuator gear is very compact in design and, when reduction is high, can produce very strong actuating forces with a high degree of positional accuracy.
In order to reduce the friction and wear between the ends of the transfer element and the spiral band, the ends are advantageously outfitted with sliding blocks or rollers that are supported on the ends by means of plain bearings or rolling bearings. Moreover, to promote good guidance of the spiral band, the ends are arranged on the circumference in pairs, whereby one spindle of a pair of spindles is offset in relation to the other spindle in the axial direction and in the circumferential direction, so that the spindles of one pair of spindles are located in the direction of the upward pitch of the spiral band.
The clutch servo unit typically works against the force of one or more clutch springs. Since the actuator gear is self-inhibiting, actuating forces are supported by the housing via the gear elements, so that the electric motor need only supply the forces required to move the throwout bearing. In order to keep these forces as low as possible and to reduce themxe2x80x94to the greatest extent possiblexe2x80x94to the level needed to overcome the frictional forces, an accumulator spring acts on the axially displaceable gear element. As a result of this, the forces of the clutch springs are offset. Since the accumulator spring is supported on the one hand by the housing and, on the other, by the axially-moveable, rotating gear element, it is advantageous to provide an axially-acting rolling bearing between the accumulator spring and the axially displaceable gear element. Accumulator springs are known in the technical field, of course, and, in fact, in different mechanical and hydropneumatic embodiments.